Use of synthesised polymers for the development of new osmotic tensiometers

Abstract

Compared with conventional water-filled tensiometers, polymer-filled osmotic tensiometers (OTs) can measure a higher matric suction by increasing the osmotic pressure of the water inside the tensiometer. The measuring range of an OT is equal to the osmotic pressure of water inside the OT. All previous research has used commercial polymers in developing OTs and the mechanism of long-term pressure decay of OTs was not found. In this study, polymers including polyacrylamide (PAM) and sodium polyacrylate (NaPA) with different degrees of cross-linking were synthesised by ultraviolet polymerisation to develop new OTs. Long-term pressure variations of OTs indicated that the pressure decay was more likely due to the stress relaxation of polymer materials instead of polymer leakage. PAM-filled OTs have a faster stress relaxation behaviour than NaPA-filled OTs, so that they can keep constant pressure (i.e. constant measuring range) for a long time and there is no need for pressure correction based on the pressure decay rate. However, the measuring range of PAM-filled OTs is lower than that of NaPA-filled OTs. The result of matric suction measurements demonstrated that the proposed OTs can be used to measure matric suction both in low and high ranges with fast response and high accuracy.

Use of osmotic tensiometers in the determination of soil-water characteristic curves

“…The pressure transducer had a measuring range up to 25bar. The crosslinked sodium polyacrylate (NaPA) with a 10% degree of crosslinking was used for the preparation of OT because of its good performance in the previous study (Liu et al, 2022b).…”

“…The use of a 15-bar ceramic disk can help to avoid the potential water loss from the saturated ceramic disk during the soil suction measurement as the soil suction is lower than the air-entry value (AEV) of the ceramic disk (Fredlund et al, 2012;Ridley, 2015). The detailed preparation procedure of OTs followed that of the previous study (Liu et al, 2022b). After the preparation of OT, it was placed in distilled water for pressure observation.…”

“…10 summarizes all the results of soil suction measurement using OT. The change in the pressure of OT during the measurement was converted to the measured matric suction following the method proposed by Liu et al (2022b). The pressure decay and the temperature effect on the measurement have been calibrated based on parameters in Table 3 and Eq.…”

“…Compared with HCTs, osmotic tensiometers (OTs) can measure a high soil suction by using polymer to increase the osmotic pressure of the water inside the tensiometer (Bocking and Fredlund, 1979;Bakker et al, 2007;Liu et al, 2022a;Liu et al, 2022b). This increase in the osmotic water pressure is regarded as the measuring range of the OT.…”

“…This increase in the osmotic water pressure is regarded as the measuring range of the OT. Cross-linked super water-absorbent polymers have been proved to be suitable for the development of OTs (Biesheuvel et al, 2000;Liu et al, 2022b). Compared with HCTs, the preparation procedure of OT is simpler and the pre-pressurization of water inside the tensiometer is not required.…”